The invention relates to downhole anchoring tools conveyed by non-rigid carriers, such as wirelines or slicklines.
To complete a well, one or more formation zones adjacent a wellbore are perforated to allow fluid from the formation zones to flow into the well for production to the surface. A perforating gun string may be lowered into the well and the guns fired to create openings in casing and to extend perforations into the surrounding formation.
For higher productivity, underbalanced perforating may be performed in which the pressure in the wellbore is maintained lower than the pressure in a target formation. With underbalanced perforating, formation fluid flow can immediately begin to enter the wellbore. The pressure difference between the formation and the wellbore in the underbalance condition may help clear the perforations by removing crushed rock, debris, and explosive gases from the formation. However, perforating in an underbalance condition may cause a sudden surge in fluid flow from the formation into the wellbore, which may create a pressure impulse that causes movement of the perforating gun string, particularly if the gun string is carried by a non-rigid carrier such as a wireline. If the pressure impulse from the surge is large enough, the perforating gun string and associated equipment may get blown up or down the well, which may cause the perforating gun string to be stuck in the well because of entanglement with cables and other downhole equipment. The shock created by the pressure impulse may also cause the perforating gun string to break from its carrier. Pressure impulses may also be caused by other conditions, such as when valves open, another perforating gun is fired, during gas (propellant) fracture stimulation, and so forth.
To address the problem of undesired movement of perforating gun strings, "reactive" anchors have been used. Such relative anchors are actuated in response to pressure impulses of greater than predetermined levels that cause acceleration of the anchor. In response to greater than predetermined acceleration, the anchor sets to effectively provide a brake against the inner wall of the wellbore to prevent the perforating gun string from moving too large a distance.
However, a disadvantage of such anchors may be that, although movement is limited, undesirable displacement may still occur in the presence of pressure surges from various sources in a wellbore. Such displacement may cause a perforating gun string to be moved out of the desired depth of perforation. A surge in fluid flow may occur during draw down of a wellbore to an underbalance condition. To reduce the pressure inside the wellbore relative to the formation pressure of a first zone, a second zone may be produced to create a rapid flow of fluid in the wellbore to the surface to lower the wellbore pressure. If the initial pressure surge due to production from the second zone is large enough, a perforating gun string located in the wellbore may be displaced a certain distance before a reactive anchor connected to the gun string is able to stop the string.
Another disadvantage of reactive anchor systems may be that they are responsive only to force applied from one direction. Thus, such anchors may not actuate in response to a pressure surge from an opposite direction. A further disadvantage may be that such anchors are not positively retracted.
Another type of anchor device is one which is set and released by cycling the wireline or slickline up and down. These types of devices typically employ a "J"-slot type mechanism which allows cycling of the anchor section from the set position to the release position. The problem with these devices is that they do not operate reliably at high angles of wellbore inclination (e.g., &gt;45 degrees). The problem is accentuated more when the well has a tortuous trajectory which makes operating any device by means of cable movement impractical.
Thus, an improved anchoring method and apparatus is needed for use with downhole tools such as perforating gun strings.